Server on an isolate, dedicated server binary, room -> server (proto 5)

- serve(Map config, events) is the one server entrypoint: the app
  hosts it via Isolate.spawn (crash surfaced on the Host card, logs
  forwarded as [server]); bin/server.dart is the dedicated server,
  compiled into the bundle as watch-together-server. --headless gone.
- Host identity via a per-hosting hostKey in hello, replacing the
  loopback heuristic.
- Rename room -> server across code, files and wire (t:"state",
  /health name); protocol bumped to 5.
- CI compiles the server binary; isolate smoke tests in server and
  desktop suites. Docs updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/HOW-IT-WORKS.md

@@ -33,8 +33,8 @@ A five-minute tour. Details: [ARCHITECTURE.md](ARCHITECTURE.md), wire format:
 
 - **Host from the app**: the app you click Host in also runs the server.
   Simplest. The server lives as long as that app.
-- **Dedicated server**: a separate server program on an always-on box.
-  Planned, see [ROADMAP.md](ROADMAP.md).
+- **Dedicated server**: `watch-together-server`, a separate program in the
+  release tarball, for an always-on box. See the README.
 
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